I am an audiologist working for a London hospital and am interested in trying
to computerise some of the analyses of tests we currently work out by hand.
One test involves playing speech at different intensity levels and then
plotting the patients performance as a series of percentages on a graph.

E.g. at 10dB 20%, 20dB 40%, 45dB 60% etc.
A line of best fit is then drawn and the resulting graph used to obtain data
to put through some formulae. I can quite easily use Excel to plot an
appropriate chart and add a suitable trendline but I am at a loss ias to how
to get excel to "read" data from that trendline.

For example one of the formulae we use is to determine what we call the
speech reception threshold, the intensity of sound at which the Pt can
correctly repeat speech 50% of the time. This intensity rarely corresponds to
an actual intensity that has been presented but is simply obtained by tracing
across from 50% on the Y axis until the trendline is met and then reading
down from that point to the corresponding intensity level on the x axis.

I have no idea how I can get Excel to go through this process or even if it
can!
I would be extremely grateful for any help as now I've started doing this
I'd like to see it through to completion.

Thank You
AB